There is a demand for increasingly accurate and precise measurements of energy consumption inside combustion chambers and reactors. To achieve these goals, the sensing devices must interact directly with the sensing environment, and the data obtained must be transmitted with minimum distortion through interface circuitry to a measurement device. Since typical temperatures in such chambers usually exceed 125 degrees C., device reliability and survivability is questionable for most conventional piezo-resistive sensors.
Efforts have been made to increase the effective operating temperature of piezo-resistive pressure sensors. Petersen et al. describe a piezo-resistive, silicon-based pressure sensor that is operable at temperatures as high as 250.degree. C. Such operation is made possible through the use of a silicon fusion bonding process in which resistors from one wafer are bonded to an oxidized surface of a second wafer. The intermediate oxide layer electrically isolates the resistors and provides a high-temperature capability. The Petersen et al. work is reported in "Ultra-Stable, High-Temperature Pressure Sensor Using Silicon Fusion Bonding," Sensors and Actuators, A21-A23 (1990) pages 96-101.
Substantially all reported sensor devices for high temperature applications still retain parameters that are sensitive to the high temperatures. In such devices, problems of redistributed impurity concentration profile, leakage current, and strain-inducing thermal coefficient mismatches have been reduced, but not eliminated.
Applicants have previously disclosed a silicon-based pressure sensor suitable for operation in a high-temperature environment of 650.degree. C. (see "An Inductively Coupled High Temperature Silicon Pressure Sensor", Proceedings of the Sixth Conference on Sensors and Their Applications, 15 Sep. 1993, edited by Grattan, pages 135-140). FIG. 1 hereof is a sectional diagram of the high temperature pressure sensor as aforedescribed. The starting material for the sensor device was a silicon wafer 10, into which a cavity was anisotropically etched.
A silicon dioxide layer 12 was grown on the etched wafer, followed by the sputtering of a tantalum silicide layer over the oxide. The tantalum silicide layer was then masked and dry etched to form a secondary coil 14. Thereafter, silicon nitride was deposited to isolate the coil windings. The cavity was then filled with sacrificial phospho-silicate glass and planarized at the rim. A further nitride layer was deposited on the sacrificial phospho-silicate glass and a tantalum silicide conductive layer deposit sputtered thereon. Thereafter, the tantalum silicide layer was masked and etched to form a primary winding 16 which was subsequently insulated by an additional nitride layer. A one-micron polysilicon layer 18 was then deposited to form a diaphragm over primary winding 16. Thereafter, the phospho-silicate glass was etched to form cavity 20.
While the structure shown in FIG. 1 operated satisfactorily, it was found that thermal co-efficient of expansion differences between silicon wafer 10 and polysilicon layer 18 created problems at high operating temperatures. More specifically, at high temperatures, differential expansion between silicon wafer 10 and polysilicon layer 18 caused a buckling of polysilicon layer 18.
It is an object of this invention to provide an improved high-temperature pressure sensor wherein thermal mismatches are substantially avoided.
It is another object of this invention to provide a high-temperature pressure sensor which is adapted to sense differential pressures as well as absolute pressures.